The importance of automatic identification systems increases particularly in the service sector, in the field of logistics, in the field of commerce and in the field of industrial production. Thus, automatic identification systems are implemented more and more in these and other fields and will probably substitute barcode systems in the future. Further applications of identification systems are related to the identification of persons and animals.
In particular contactless identification systems, like transponder systems for instance, are suitable for a wireless transmission of data in a fast manner and without cable connections that may be disturbing. Such systems use the emission and absorption of electromagnetic waves, particularly in the high frequency domain. Systems having an operation frequency below approximately 800 MHz are frequently based on an inductive coupling of coils, which are brought in a resonance state by means of capacitors, and which are thus only suitable for a communication across small distances of up to one meter.
Due to physical boundary conditions, transponder systems having an operation frequency of 800 MHz and more are particularly suitable for a data transfer across a distance of some meters. These systems are the so-called long-range RFID-systems (“radio frequency identification”). Two types of RFID-systems are distinguished, namely active RFID-systems (having their own power supply device included, for example a battery) and passive RFID-systems (in which the power supply is realized on the basis of electromagnetic waves absorbed by an antenna, wherein a resulting alternating current in the antenna is rectified by a rectifying sub-circuit included in the RFID-system to generate a direct current). Moreover, semi-active (semi-passive) systems which are passively activated and in which a battery is used on demand (e.g. for transmitting data) are available.
A transponder or RFID tag comprises a semiconductor chip (having an integrated circuit) in which data may be programmed and rewritten, and a high frequency antenna matched to an operation frequency band used (for example a frequency band of 902 MHz to 928 MHz in the United States, a frequency band of 863 MHz to 868 MHz in Europe, or other ISM-bands (“industrial scientific medical”), for instance 2.4 GHz to 2.83 GHz). Besides the RFID tag, an RFID-system comprises a reading device and a system antenna enabling a bi-directional wireless data communication between the RFID tag and the reading device. Additionally, an input/output device (e.g. a computer) may be used to control the reader device.
The semiconductor chip (IC, integrated circuit) is directly coupled (e.g. by wire-bonding, flip-chip packaging) or mounted as a SMD (“surface mounted device”) device (e.g. TSSOP cases, “thin shrink small outline package”) to a high frequency antenna. The semiconductor chip and the high frequency antenna are provided on a carrier substrate that may be made of plastics material. The system may also be manufactured on a printed circuit board (PCB).
In order to increase the efficiency of such a transponder, an efficient antenna should be used. Further, the reflection of energy between the antenna and the semiconductor chip should be as low as possible. This may be accomplished by matching the electromagnetic properties of the semiconductor chip and the electromagnetic properties of the antenna. A maximum amount of power may be transmitted, if the value of the impedance of the semiconductor chip Zchip is complex conjugate to the value of the impedance of the antenna Zant:Zchip=Zant  (1)Rchip+jXchip=Rant−jXant  (2)
In equation (2), Rchip denotes the ohmic resistance of the semiconductor chip, j is the imaginary number, and Xchip is the (inductive or capacitive) reactance of the semiconductor chip. Rant is denoted the ohmic resistance of the antenna, and Xant is the (inductive or capacitive) reactance of the antenna.
As can be seen from equations (1) and (2), for an appropriate impedance matching, the absolute values of the real parts of the complex impedances of the semiconductor chip and of the antenna should be equal, and the absolute values of the imaginary parts of the complex impedances should be identical, wherein the reactance of the semiconductor chip should be complex conjugate to the reactance of the antenna.
According to the manufacturing process of a semiconductor chip, the impedance of a semiconductor chip is usually dominated by the capacitive contribution, i.e. the imaginary part Xchip is usually negative. Consequently, for an efficient transponder antenna design, the reactance of the antenna should be dominated by the inductive contribution, i.e. the reactance Xant should be positive, and its absolute value should be equal to the imaginary part of the impedance of the semiconductor chip. If this is the case, and if the condition is fulfilled that the two real parts Rchip and Rant are equal, then an efficient power matching is realized and a high energy transfer between the semiconductor chip and the antenna can be obtained. Thus, for an efficient antenna design, the real part and the imaginary part of the impedance of the antenna should be matched to a given impedance of a semiconductor chip.